Organic light-emitting diodes (OLEDs) are current-driven self-luminous devices. OLEDs have short response times, high display brightness level, high display contrasts, and wide viewing angles. OLEDs can be fabricated on flexible/soft substrates. Because of the features described above, OLEDs have been widely used in display technology. Each pixel on an OLED display panel includes OLEDs. Based on the driving method, OLED display panels may be divided into active OLED display panels and passive OLED display panels. In an active OLED display panel, a thin-film transistor (TFT) circuit may be used to control the electric current flow through each OLED such that the OLED display panel has a uniform brightness level. Each TFT in the TFT circuit needs to be sufficiently stable to ensure the current flow through the OLED remains stable.
The stability of a TFT may be susceptible to the threshold voltage of the TFT. For example, the threshold voltage of a TFT may be subjected to factors such as the doping material of the drain, the thickness of the dielectric layer, gate material, excess charges in the dielectric layer, etc. Thus, under existing fabricating conditions, the threshold voltages of the TFTs in a TFT circuit are likely to be different due to the factors described above. The differences in the threshold voltages may cause the current flowing through each OLED to vary. Therefore, pixel compensating circuits have been used to reduce the differences in the threshold voltages among the TFTs.
FIG. 1 shows an existing pixel compensating circuit. The pixel compensating circuit includes an OLED D1, a driving transistor M1, a data-voltage writing module (transistor M5), a lighting-control module (transistor M3), a switching module (transistor M2), a resetting module (transistor M4, transistor M11, and capacitor C1). In the resetting module, the capacitor C1 is connected to the power supply VDD through one terminal and connected to the reset-control signal Vreset and the initial voltage signal Vini through transistor M11. In a resetting phase, the reset-control signal Vreset is turned on, the voltage provided by Vreset remains unchanged, and transistor M4 remains on. Thus, by controlling the voltage provided by the initial voltage signal Vini, the control terminal of driving transistor M1 may be reset to a low potential Vini. As shown in this layout, the existing pixel compensating circuit requires 6 transistors, 1 capacitor, and 6 signal lines.
As described above, the structure of the existing pixel compensating circuit is undesirably complex. That is, besides the OLEDs, the existing pixel compensating circuit requires 6 transistors, 1 capacitor, and 6 signal lines. The number of transistors and the number of signal lines may both be large. The large number of transistors and signal lines may not be suitable for the layouts of display products with high resolutions, and the production cost of the display apparatus may be high.